Corrosion

Environmental testing

 

  • Important tool in the field of corrosion analysis
  • Investigation of influences like temperature, humidity and (toxic) gases on degradation and corrosion of el. devices and materials
  • Simulation of various fields of applications with their different conditions (e.g. offshore, aerospace)

Testing methods (performed at Fraunhofer IISB)

  • Salt spray (e.g. DIN EN 60068-2-52) or damp heat testing (e.g. DIN EN 60068-2-67), thermal shock, temperature cycling (e.g. DIN EN 60068-2-14)
  • Corrosive gas (H2S, NO2, Cl2, SO2 and mixed gas) (e.g. DIN EN 60068-2-42)
  • Highly accelerated stress test (HAST), pressure cooker test (PCT) (e.g. DIN EN 60068-2-67)
  • Combined with voltage treatment (power cycling) (e.g. DIN EN 60068-2-67)

Electrochemical corrosion of power electronics is in the focus
because of

  • Steadily increasing demands in terms of higher packaging densities
  • Demand for applications of power electronic modules under extreme environmental conditions is rising
  • Assemblies that are exposed to changing environmental influences e.g. in automotive, aerospace, telecommunications
  • Electrochemical migration (ECM) that leads to dendrite formation is one important  form of corrosion in power electronics
© Fraunhofer IISB

Removal of mold compound by etching

© Fraunhofer IISB

Cross section of IGBT power module

Mechanisms of ECM

  • Potential and humidity between metallic structures is present
  • Metal ions dissolve
  • Positive metal ions migrate from the anode to the cathode
  • Ions are captured at the cathode
  • Dendrites grow from the cathode to the anode

ECM occurs immediately in electronic packages if the following is given

  • Gaps exist (due to delaminated insulating potting material)
  • Metals and metal combinations tend to corrode and form dendrites
  • Humidity is present
  • A sufficient voltage load is given (several volts)

Corrosion protection

  • Concepts for corrosion protection
  • Cleaning, chemical corrosion inhibition and adhesion promotion
  • Protective coating: potting and parylene
  • Coating material characterization → analyzing methods

Failure analysis

  • Partial discharge measurement
  • Optical microscopy for searching for dendrites
  • Laser interferometry (for analysis of coating quality and coating thickness)
  • Cross-sectioning by sawing, grinding, polishing, as in figure 3
  • Cross-sectioning by femto-laser curing
  • Comparative tracking index
  • Scratch test
  • Decapsulation of mold compounds as well as silicone gels, e.g. as in figure 2
  • Scanning electron microscopy (SEM) and elemental analysis with energy
    dispersive X-ray spectroscopy (EDX), distribution and quantity, as in figures 5 and 6
  • Focused ion beam (FIB), high speed cutting by plasma
  • Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC)
  • Fourier-transform infrared spectroscopy (FTIR)
  • Radiography/ computer tomography
 

Environmental Testing,
Corrosion,
Failure Analysis

[ PDF 0,33 MB ]